Cooperative Guanidinium/Proline Organocatalytic Systems

Organocatalysis is nowadays recognized as the third pillar of asymmetric synthesis, standing next to metal catalysis and enzymatic transformations. Proline has shown up as an ideal organocatalyst, being inexpensive and readily available. However, this amino acid has also manifested its limitations. Compared to the chemical modification of proline, the approach through adding small hydrogen-bond-donating cocatalysts to interact with proline is particularly attractive. Various additives have been investigated to date. This chapter discloses the use of guanidinium salts as additives for proline, investigated in the course of proline-catalyzed aldol reactions.